1. Field of the Invention
The present invention relates to a magnetic encoder utilizing elements whose electric resistances vary depending upon magnetic fields according to a so-called magnetoresistance effect to convert the change of a magnetic field to an electric signal, thereby carrying out position detection, etc., and more particularly to a magnetic encoder having increased accuracy without noises in detection signals.
2. Description of the Related Art
In a wide variety of fields, such as NC machines, robots, OA equipment, VTRs, etc., position sensors or rotary sensors having a high degree of accuracy are needed. These sensors are used in various servomotors, rotary encoders, etc. in the above-noted industrial fields. Also, as the technologies of OA equipment and factory automation develop, there is an increasingly higher demand for speed and reliability in these sensors. Under such circumstances, optical-type sensors, for instance have been widely used as rotary sensors. However, they have semiconductor elements such as photocells, LEDs, etc. as constituent elements, and they are extremely sensitive to dust, and temperature variations. In addition, the optical-type sensors inevitably have a large number of constituent parts, leading to complicated structures.
Thus, magnetic-type sensors utilizing the magnetoresistance effect have recently been developed. These magnetic-type sensors show high detection accuracy and are stable to temperature variations and less sensitive to dust, so that wide ranging applications for these sensors have been developing in many fields.
FIG. 6 is a perspective view schematically showing one example of a conventional magnetic-type rotary sensor. In this figure, a magnetic drum 61 is fixed to a shaft 62 which rotates, for instance, counterclockwise in the direction shown by the arrow. The magnetic drum 61 is provided, on its side surface, with a magnetic signal-generating means 65 which has N magnetic poles arranged and S magnetic poles alternately. This magnetic signal-generating means 65 is made of a magnetic coating of .gamma.-iron, etc. or a plastic magnet of strontium ferrite or barium ferrite, etc. The number of magnetic poles in the magnetic signal-generating means 65 may be properly determined within the range of two or more, depending upon the required detection precision. Accordingly, in a case where the number of magnetic poles is as many as several hundreds to several thousands, the magnetic pole pitch of the magnetic signal-generating means 65 is inevitably extremely small.
A magnetic sensor 63 is provided with a magnetoresistance element 64 including a plurality of thin, ferromagnetic layer patterns 66 having a magnetoresistance effect formed on a glass substrate 67. The magnetic sensor 63 is disposed opposite to the magnetic signal-generating means 65 of the magnetic drum 61, with such a small gap G therebetween that a magnetic field of each magnetic pole can sufficiently reach the magnetic sensor 63.
According to the above structure, when the magnetic drum 61 is rotated, the magnetoresistance element 64 disposed opposite to the magnetic drum 61 is subjected to a periodically changing magnetic field leaked from the surface of the magnetic signal-generating means 65. Thus, each thin, ferromagnetic layer pattern (magnetic field-detecting pattern) 66 in the magnetoresistance element 64 converts the intensity of the leaked magnetic field to a change in a resistivity, thereby generating an electric signal. As a result, a rotation speed or a position can be detected.
As described above, the conventional magnetic encoder shown in FIG. 6 includes the magnetic sensor 63 and the magnetic drum 61 provided with a magnetic signal-generating means 65, having a predetermined gap G therebetween. The magnetoresistance element 64 of the magnetic sensor 63 includes a plurality of electrically combined magnetic field-detecting patterns. With the magnetic encoder having the above-described structure, the detection of rotation speeds of motors and the positions of movable members in robots, NC machines, etc. can be carried out at a high precision.
However, increasingly wider varieties of functions and higher precision are recently required for the above machines, and detection equipment is required to be smaller and more accurate. For instance, magnetic encoders capable of carrying out detection at both a high rotation speed and a lower rotation speed with high precision are required. To meet these requirements, it is necessary to generate an electric signal or an output signal having different pulse numbers. However, since most of the conventional magnetic encoders are provided with a series of magnetic poles on side surfaces of rotational drums, they fail to meet the above requirements. Also, if a plurality of magnetic encoders are combined to meet the above requirements, a large number of parts are required, and the overall structure of the apparatus would become complicated, failing to meet the demand of miniaturization.
To solve these problems, investigation has been made to provide a structure shown in FIG. 7. This magnetic encoder comprises a magnetic drum 71 provided on its side surface with a Z phase element for generating a reference signal, and increment phase elements I.sub.1, I.sub.2, I.sub.3 having a plurality of magnetic poles at different magnetic pole pitches, and a magnetic sensor 72 provided with independent magnetoresistance elements 72a, 72b, 72c disposed opposite to these increment phase elements I.sub.1, I.sub.2, I.sub.3.
With such a structure, output signals having different pulse numbers can be generated. If an output signal having a lower pulse number is used for detection at a higher rotation speed, and an output signal having a higher pulse number is used for detection at a lower rotation speed, high-precision detection can be achieved at both higher and lower rotation speeds. Also, by simultaneously detecting the low-pulse number signal and the high-pulse number signal, a signal pulse of high precision can be synthesized in a subsequent circuit.
However, in a case where the increment phase elements I.sub.1, I.sub.2, I.sub.3 having different magnetic poles are disposed on the same magnetic drum, the following problems arise. That is, for instance, in a case where the increment phase elements I.sub.1, I.sub.2, satisfy the relation l.sub.1 /l.sub.2 &gt;1, wherein l.sub.1 and l.sub.2 represent the magnetic pole pitches of the increment phase elements I.sub.1, I.sub.2, a gap G between each of the increment phase elements I.sub.1, I.sub.2, and each of the corresponding magnetoresistance elements 72a, 72b results in the relation shown in FIG. 8 between the output of each magnetoresistance element 72a, 72b and the size of the gap G.
When there is a large magnetic pole pitch, the magnetic signal supplied from the magnetic drum 71 is strong in a wide range of gap, with a small variation of an output relative to the variation of the gap. On the other hand, when the magnetic pole pitch is small, the magnetic signal is weak, and its strong region is narrow on the side of the smaller gap, the output drastically changing relative to the gap variation. Accordingly, the gap between the magnetic drum 71 and the magnetic sensor 72 should be such that it is suitable for the magnetic signal-generating means having a smaller magnetic pole pitch. One example of such a small gap is shown as "G" in FIG. 8.
The inventors have been investigating a magnetic drum having different-diameter areas, a magnetoresistance element having a larger magnetic pole pitch provided on a larger diameter area of the magnetic drum, and a magnetoresistance element having a smaller magnetic pole pitch provided on a smaller diameter area of the magnetic drum, thereby providing two signals on the same drum surface.
However, the signal generated by the magnetoresistance element corresponding to the magnetic signal-generating means having a larger magnetic pole pitch is a deformed waveform containing noises as shown in FIG. 9. This seems to be due to the fact that since the gap between the magnetic drum and the magnetic sensor is determined such that it is proper for the magnetoresistance element opposite to the magnetic signal-generating means having a smaller magnetic pole pitch (smaller gap), the gap is not necessarily suitable for the magnetoresistance element opposite to the magnetic signal-generating means having a larger magnetic pole pitch. Further, while the magnetic field-detecting patterns of the magnetic sensor can occupy a considerable width in the case of the smaller magnetic pole pitch, the percentage of the pattern width in the magnetic field is small in the case of the larger magnetic pole pitch, whereby the magnetic sensor is more sensitive to the fluctuation of the magnetic field.
In general, increment signals (INS) having a phase difference of 90.degree. are evaluated with respect to their precision of waveforms by forming a Lissajous figure. As is shown in FIG. 10A, when signals A and B having a phase difference of 90.degree. are plotted as an X coordinate and a Y coordinate, a circular Lissajous figure as shown in FIG. 10B can be obtained. When the signals A, B are completely sine and cosine curves, the resulting Lissajous figure is completely circular. However, in the conventional magnetic encoder, deformed waveforms are generated, resulting in a deformed circle as shown in FIG. 11.